


Detention

by orphan_account



Series: Nagisa and Korosensei [1]
Category: Assassination Classroom
Genre: Anal Sex, Boys in Skirts, Cock Rings, Cock Slut, Detention, Drooling, M/M, Nipple Play, Orgasm Delay, Rough Sex, Sexual Fantasy, Teacher-Student Relationship, Tentacles
Language: English
Status: Completed
Published: 2019-06-10
Updated: 2019-06-10
Packaged: 2020-04-24 08:56:52
Rating: Explicit
Warnings: Underage
Chapters: 1
Words: 1,777
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/19169962
Author URL: https://archiveofourown.org/users/orphan_account/pseuds/orphan_account
Summary: Nagisa is stuck in detention with only his sexual fantasies to keep him occupied. When Koro-sensei catches on, things get a little heated.





	Detention

**Author's Note:**

> The only thing I know about smut is what i've learned from reading smut.

Nagisa stares out the window watching his classmates head home for the day wishing that he wasn't stuck in detention for failing his English test. Koro-sensei rarely gave out detentions, but Karasuma-sensei was pushing him to be a little more stern with his students after they pranked him last week with water balloons. Now Nagisa regrets taking part in that prank, and forgetting to study his English vocabulary in favor of planning it. 

Nagisa glances at the front of the classroom. Koro-sensei is quietly grading homework at a slow pace instead of at mach speed like usual to keep himself occupied during the hour he had to guard Nagisa. Nagisa doesn't know this, but Koro-sensei is wishing he could read the porn mags stashed in his desk.

Sighing, Nagisa closes his eyes and puts his head on his arms. Detention is so boring. What is he supposed to do? Maybe he could plot assassination attempts in his head. For five minutes, he does just that. As he ran through various assassination scenarios, intrusive thoughts begin to invade his mind. 

For example, Nagisa was imaging pining Koro-sensei into a corner with a knife against his throat, when all of a sudden he was imagining Koro-sensei reversing their positions and fucking him against the wall. Nagisa blushes and hides his face.

Nagisa has known for a while that he is attracted to Koro-sensei. He doesn't understand why and is embarrassed by his hidden feelings. Every-time Koro-sensei so much as glances in Nagisa's direction, he can't help blushing as his heart flutters in his chest. Now that he is alone in a room with Koro-sensei, all he could think about is how it would feel to have tentacles coiling around his naked body. Nagisa sighs as his mind comes up with another sexy scenario.

The scenario goes like this:

Nagisa enters the room confidently dressed in school girl's uniform (he knows Koro-sensei has a fetish for those). The skirt is skimpy and if Nagisa were to bend down his ass would be visible to anyone happening to look. The top buttons on his uniform are undone revealing his smooth white chest and perky nipples. Although Nagisa knows that in reality he'd never be so confident. 

"Hello, Nagisa. Do you need something?" Says Koro-sensei, without looking up. Nagisa smirks. 

"Yes, I need you." Nagisa catches Koro-sensei's attention. Suprised, Koro-sensei stares at Nagisa, eyes sliding down his slim figure and settling on his crotch. The tip of Nagisa's length is just barely peeking out beneath the hem of his skirt. Shutting the door, he moves slowly towards the desk while twirling a lock of hair. "Sensei~" Nagisa purrs, leaning over Koro-sensei's desk and grabbing his tie. Faces inches from each other, Nagisa blinks at him with innocent eyes. "Would you mind tutoring me?"

"Tutoring you in what?" Koro-sensei stutters, his round face turning red. 

Nagisa grins coyly fingering the hem of skirt. "Sexual education. I find myself lacking in experience and I would love for you to show me some tricks." 

Koro-sensei hesitates, licking his lips nervously. "I'd be happy to help. Why don't you undress for me so we can get started?" 

Suddenly, Nagisa's fantasy is interrupted by a warm, tight feeling in his crotch. Looking down, he sees a bulge appear in his pants. Oh no. He had gotten himself hard. Stealing a look at Koro-sensei, Nagisa sticks his hand in his pants and lightly presses the bulge in a vain attempt to calm himself down. He can't let Koro-sensei notice. 

"Playing with yourself, Nagisa-kun?" Koro-sensei is behind him whispering into his ear. "What a naughty boy." 

Nagisa jumps, face turning redder than a firetruck. "It's not what you think!" He stammers, removing his hand from his pants. How did Koro-sensei notice?! "I didn't mean to!" 

"Don't worry, I know all about a pre-teen boy's urges." Koro-sensei chuckles. "Allow me to help relieve you." 

Nagisa's breath hitches. "What?" 

Without warning Koro-sensei's tentacles are wrapped all around Nagisa's body burrowing under his shirt and into his pants. His tentacles feel cool and Nagisa shivers as they slide against his skin and tickle his thighs. 

"S-sensei what are you doing?" Nagisa gasps as Koro-sensei's begins to fondle his sensitive nipples. His cock twitches and strains against his pants. 

"What does it look like i'm doing?" Koro-sensei grins evilly, groping Nagisa's butt cheeks with his hands. "I'm helping a student. Am I making you uncomfortable?"

Nagisa shakes his head frantically. "N-no, of course not!" The feeling of Koro-sensei's tentacles caressing him is positively euphoric and his cock is begging to be freed from its confines. He throws his head back and moans when a teasing tentacles begins to slid up his ass. 

"Good. Then I guess you won't mind if I remove your clothes then, right?" 

Nagisa doesn't even have a chance to answer before he's stripped of his clothing in mach speed. He's now sitting in his chair completely exposed in front of his crush with his cock flicking about wildly. Trembling in embarrassment, he halfheartedly attempts to cover his obvious erection. 

"Don't be shy." Koro-sensei says soothingly, pulling Nagisa's hands away. Nagisa's cock springs into the air again leaking pre-cum. It's longer than Koro-sensei expected and decently thick too. Chuckling again, Koro-sensei wraps a tentacle around Nagisa's length and begins to expertly pump his cock eliciting another cute moan. Another tentacle cups his balls and squeezes them making Nagisa's moans louder. Koro-sensei simultaneously tickles Nagisa's prostate. 

"Ah! Sensei, that feels good~" Nagisa mewls, squirming in his seat. Another tentacles slides up his ass. 

"Such a cute little penis. And what a round, perfect butt you have." Koro-sensei squeezes Nagisa's cock and rubs his erect nipples. "Tell me, Nagisa-kun. Have you ever been fucked before?" 

Nagisa can only manage a nod as a third tentacle slips inside him. He closes his eyes and continues to moan from Koro-sensei pumping his cock up and down and stretching his ass. Nagisa hips buck trying to get more friction against his cock and he begins to grind in his seat. 

"There's a first time for every thing." Koro-sensei gently forces Nagisa to stand up and positions him so that he's propping himself up against the desk with his ass completely in view. He licks his lips again at the sight of his ass twitching and waiting to be penetrated. 

Koro-sensei continues to prepare the young boy for a moment enjoying his sweet moans. He can tell the boy is close to cumming and squeezes the base of his cock to prevent it. Removing Nagisa's hair ties he uses them as makeshift cock rings to replaces his tentacles. 

"Sensei! I need to cum!" Nagisa pants. "I need you to..." 

"I need you to what?" Koro-sensei asks, as he pours lube into Nagisa's hole. He's generous with the lube emptying the entire bottle and spreading it around Nagisa's cheeks. Nagisa waves his ass enticingly. 

"I need you to fuck me." He whispers, biting his lip. "Please!" 

"Are you sure, Nagisa-kun?" Koro-sensei removes his tentacles and Nagisa whines from the loss. The boy is properly prepared and Koro-sensei can't wait for the real fun to begin.

"Yes! Please! It's all I've ever wanted!" Lube drips down Nagisa's trembling thighs. He spreads his legs even wider pleading for his teachers cock. 

Koro-sensei smirks. "Then will you say that you want my cock in your ass fucking you without mercy? Will you promise to be my little slut from now on to please me whenever I call?" He continues to caress Nagisa's body and tease his welcoming entrance pushing him over the edge. Nagisa's cock bounces against his belly. 

"Yes! I want your cock in my ass fucking me without mercy! I promise to be your little slut and I'll come to you whenever you want so please-! Please put your dick inside me!" Nagisa is practically crying at this point, his tears falling on the desk and his body shaking with anticipation. He doesn't even notice that he's drooling. A thin trail of saliva drips down his chin. 

"My, my. Such a needy student." Koro-sensei obliges and begins to push his cock into Nagisa. Nagisa stiffens as Koro-sensei moves in painfully slow inch by inch. Koro-sensei's cock is thick and huge and feels to Nagisa like it must be at least seven inches long. He doesn't relax until Koro-sensei's thick cock is completely buried inside of him making his ass full. 

"Uwah~!" Nagisa's body is struck with pleasure. "Sensei, your cock feels so good!" He grinds his ass against Koro-sensei.

Koro-sensei begins to thrust at a steady pace making Nagisa moan's echo against the walls of the empty classroom. He picks up the pace and soon Nagisa is practically howling with delight as Koro-sensei rams into his hole over and over again. 

"More, sensei! More!" Nagisa cries, his cock bouncing faster against him. "Fuck me harder!" Koro-sensei thrusts faster to the point of abuse. 

"Such a little slut! I didn't expect this from you, Nagisa!" Koro-sensei's tentacles tighten around Nagisa to keep him steady. "I wish I knew sooner how much you wanted my cock!" 

Koro-sensei relentlessly fucks Nagisa for the rest of the hour holding back his own release to make his fun last. He relishes in the heat of the boy's walls rubbing on his erection as he slides in and out. Finally, when the hour is up, Koro-sensei removes the cock ring from Nagisa and his seed shoots onto the desk prompting a scream of relief to escape Nagisa lips. With a few more quick thrusts, Koro-sensei cums inside Nagisa making the boy feel every bit of his orgasm as his cock spasms inside him. He pulls out slowly enjoying the feeling of Nagisa's walls clenching against his cock. 

"How was that for your first time?" Koro-sensei asks once his cock is out completely. Semen and lube drip onto the floor. 

"That-that was amazing!" Nagisa says breathlessly, "It was everything I've ever wanted!" 

"I'm glad I could help you out, Nagisa." Koro-sensei starts to clean up the boy and redress him. 

"Sensei, did this really happen? Or was it all a dream?" Nagisa asks, staring into Koro-sensei's eyes.

Koro-sensei throws his head back and laughs. "Oh, it was all real alright." 

Nagisa beams. "Then can we do this again sometime?"

"Of course we can! You promised to be my slut, remember?" Koro-sensei says seriously. 

"I remember. Just, will you call all me again soon...?" 

Koro-sensei gives a sly grin. "Well I have to punish you for making a mess of the classroom." He finishes buttoning Nagisa's shirt and caresses his face. "Detention. Tomorrow. Don't be late."

**Author's Note:**

> Ah, what have I done.


End file.
